Reflected light undergoes a phase-shift that varies with the material composition of the reflecting surface. For brevity this phase change will be labelled the Phase Shift on Reflection (PSoR). At a dielectric-dielectric boundary a normally incident plane wave undergoes a PSoR depending on the relative refractive indices of the two materials. At an air-glass boundary for example PSoR=u. At dielectric-metal interfaces the PSoR varies with the metallic material. This shift is determined by the complex refractive index of the material (dependent upon material composition). Further variations occur with changes in wavelength of incident radiation and with the angle of the incidence of the illumination, and with changes in the relative height of the sample across a sampled area. PSoR variations due to these parameters can be a source of error for optically based, non-contact profilometers, since the optical path length of the test beam and measured fringe pattern are altered. The effect is directly analogous to an impedance mismatch causing back-reflections in an RF cable. Ellipsometry is generally used for measurement of PSoR, making use of the elliptical polarisation of light reflected from an obliquely illuminated sample. However ellipsometry measures average values over relatively large (>10 μm) illumination spot, and cannot easily produce spatial maps.
Common optical metrology methods for non-contact surface topography measurement such as scanning white light interferometry (SWLI), phase shifting interferometry (PSI) and digital holography are unable to distinguish between true height variations of a surface and phase changes caused by material variations. Typically prior knowledge of the composition of the surface is required to apply correction factors to the acquired data. In addition, they can suffer from topographic errors due to so-called 2π uncertainties where fringe order from the interferogram is mis-calculated.
It is an aim of embodiments of the present invention to at least partly mitigate the above-mentioned problems.
It is an aim of certain embodiments of the present invention to enable the determination of characteristics such as optical or dielectric properties of a target sample.
It is an aim of certain embodiments of the present invention to enable the determination of topographic properties of a target sample.
It is an aim of certain embodiments of the present invention to enable the determination of optical or dielectric properties and topographic properties of a target sample.